The invention relates to a single- or multilayer, biaxially oriented film based on crystallizable polyesters and comprising at least one primary stabilizer for inhibiting oxidative degradation. It also relates to a process for producing the film.
Films based on crystallizable polyesters, in particular based on crystallizable polyethylene terephthalates, are known. Polypropylene films which comprise additives for preventing oxidative degradation have also been described.
However, the surface quality of these known polyester films is inadequate for some applications. For example, substrate films for magnetic tapes have to have particularly uniform structure. Non-uniformity in the film structure is also very disadvantageous for capacitor films, thermal transfer films, packaging films, and films for industrial applications. The surface defects are generally brought about by gel particles and also impair appearance and further processing, for example metalizing or printing.
Polyester films are generally produced from polymer pellets melted in an extruder. The resultant polymer melt is molded by way of a slot die to give what is known as a prefilm. The prefilm is then applied to a take-off roll and chill roll, and then oriented longitudinally and transversely, and finally rolled up. These rolls of film should have no longitudinal corrugations, creases, run-outs, or other defects which impair appearance or further processing. Longitudinal corrugations in the film can generally be attributed to defective transverse profile. The transverse profile is also adversely affected by deposits on the extrusion die. Die deposits are in turn mostly produced from constituents of the polymer melt.
It was therefore an object to reduce the number of surface defects in the polyester film. In particular, the film should have fewer specks and gel-particle-based defects. In addition, the film should have no longitudinal corrugations or other defects which impair roll structure. Furthermore, process stability should be raised, i.e. no film break-offs or similar disruptions should occur. Screen service life should be prolonged, and drying times shortened, and it should be possible to use film regrind, even in relatively large proportions. Throughput and production rate should moreover be optimized.
The object is achieved if the stabilizer is added, and thus incorporated into the polyester, prior to the polycondensation process.